1. Field of the Invention
The present invention relates to the field of vehicular wheelchair lifts which enable persons who are physically challenged or otherwise have limited mobility to board and leave a vehicle. More particularly, the present invention relates to the field of flow control valves used in hydraulic systems for vehicular wheelchair lifts or the like.
2. Description of the Prior Art
Specifically, hydraulic vehicular wheelchair lifts are well known in the art. Many of these lifts utilize a hydraulic actuating system with a flow control valve for moving the lift platform between a ground level position to an entry level position and from the entry level position to a stowed position. Furthermore, these lifts are also designed to be gravity-down during the platform movements from the stowed position to the entry level position and from the entry level position to the ground level position.
One of the disadvantages with prior art flow control valves is that they are restricted in one direction. Another disadvantage is that the main orifice port which is used for restricting hydraulic fluid flow has a narrow diameter opening and can get clogged by small particles in the hydraulic fluid. A further disadvantage is that there are three fluid routes in which the hydraulic fluid flows through the flow control valve.
U.S. Pat. No. 5,639,066 issued to Lambert et al. on Jun. 17, 1997 discloses a bidirectional flow control valve. The valve regulates flow in either direction between two ports having a valve with one of the ports formed in the housing at a proximal end and coaxial with the main bore and with a second one of the ports formed in the housing and opening into a side of the bore. A tubular spool is slidably received in the bore, and the spool has a lumen which is coaxial with the first port and a radial opening in the wall of the spool, with an edge of the opening being positioned to wipe across an edge of the second port so as to vary the cross-sectional area of a flow passage between the first and second ports in an area defined between the edges of the opening and of the second port.
U.S. Pat. No. 5,308,215 issued to Saucier on May 3, 1994 discloses a hydraulic drive passenger lift with a flow control structure for accommodating various distinctive motion patterns, including lifting "up and down" (loading and unloading) and "fold and unfold" (storage and deploy) patterns. The flow control arrangement with different attenuation degrees is designed for the purpose of having similar reduced platform movement speeds during the storage phase and the deployment phase of the storage movement pattern. The flow control structure includes multiple paths between a source and an actuator, with control for selecting the appropriate path to yield predetermined flow rates. A solenoid driven spool valve and a biased movable orifice member select the desired path. While the flow is substantially unrestricted during "up and down" movement, it is somewhat restricted for reduced speed during the "unfold" movement and is more restricted during the power-driven "fold" movement.
U.S. Pat. No. 5,320,135 issued to Pierrou on Jun. 14, 1994 discloses a flow compensator valve that is used in a hydraulic system which includes a cylindrical body having inlet and outlet ports, and a slidable spring-biased hollow piston insert disposed within a center bore of the body, which insert has a plurality of ports for flow control depending on the position of the insert as it reciprocates axially within the center bore of the valve body.
It is desirable to provide a hydraulic system with an improved bidirectional flow control valve to eliminate the disadvantages mentioned above. It is also desirable to provide an improved bidirectional flow control valve in which the hydraulic flow can be restricted in either direction of flow of the valve and the relative restricted flow rates may be the same or various.